I have been a paying member of GameTZ, the Game Trading Zone, for years. Just today removed the fees and opened up the site. Considering the cost has been $5 for 2 months, this is a fairly warm deal for what you get. The best part of the site (besides the forums) is the item/offer system. You list your games available for sale and ones you want and it matches you up. You can then send an offer and negotiate, or just post what you want/have in the forums. I am a big NES collector and have purchased many, many games from this site, as well as sold some doubles. They have something for all systems. Check it out, I'm glad I stumbled across it.
http://gametz.com/
http://gametz.com/